1. Field of the Invention
The invention relates to an electroluminescent display device, particularly to a display device having a pixel selecting transistor and a driving transistor for current-driving an electroluminescent element.
2. Description of the Related Art
In recent years, an organic electroluminescent (hereafter, referred to as “EL”) display device using organic EL elements has been receiving attention as a new display device substituted for a CRT or an LCD. Particularly, an organic EL display device having thin film transistors (hereafter, referred to as TFTs) as switching elements for driving the organic EL elements is being developed.
FIG. 4 is an equivalent circuit diagram of one pixel in an organic EL display panel. In an actual organic EL display panel, a plurality of the pixels is disposed in a matrix of n rows and m columns. A gate signal line 50 for supplying a gate signal Gn and a drain signal line 51 for supplying a display signal Dm intersect each other.
An organic EL element 52, a driving TFT 53 for driving the organic EL element 52, and a pixel selecting TFT 54 for selecting a pixel are disposed on a periphery of an intersection of these signal lines.
A source of the driving TFT 53 is supplied with positive power supply voltage PVdd from a power supply line 55. A drain of the driving TFT 53 is connected with an anode of the organic EL element 52. A cathode of the organic EL element 52 is supplied with negative power supply voltage CV.
A gate of the pixel selecting TFT 54 is connected with the gate signal line 50, and supplied with the gate signal Gn. A drain 54d of the pixel selecting TFT 54 is connected with the drain signal line 51, and supplied with the display signal Dm. A source 54s of the pixel selecting TFT 54 is connected with a gate of the driving TFT 53. The gate signal Gn is outputted from a vertical drive circuit (not shown). The display signal Dm is outputted from a horizontal drive circuit (not shown).
Furthermore, the gate of the driving TFT 53 is connected with a storage capacitor Cs. The storage capacitor Cs stores the display signal Dm for the display pixel for a field period by storing electric charge corresponding to the display signal Dm.
An operation of the EL display device having the described structure is as follows. When the gate signal Gn becomes high for a predetermined horizontal period, the pixel selecting TFT 54 turns on. Then, the display signal Dm is applied from the drain signal line 51 to the gate of the driving TFT 53 through the pixel selecting TFT 54.
According to the display signal Dm supplied to the gate, the conductance of the driving TFT 53 changes. A drive current corresponding to the changed conductance is supplied to the organic EL element 52 through the driving TFT 53, lighting the organic EL element 52. When the driving TFT 53 turns off according to the display signal Dm supplied to the gate, an electric current is not supplied to the driving TFT 53 so that the organic EL element 52 also turns off the light. Such a structure is described, for example, in Japanese Patent Application Publication No. 2002-175029.
However, characteristics of the driving TFT 53, for example, a threshold voltage vary among the pixels depending on manufacturing conditions so that current-drive performance of the driving TFT 53 differs among the pixels. Therefore, a luminance of the organic EL element 52 differs among the pixels, resulting in an uneven display on an organic EL display panel.